A new study indicates that new nutrient reduction strategies could help reduce the size of the annual low-oxygen "dead zone" along Louisiana's coast. This color-coded map shows oxygen levels in bottom waters of the Gulf of Mexico off the Louisiana coast. The darkest red areas, surrounded by a black line, show where the oxygen level is below 2 parts per million, which is considered hypoxia or the "dead zone."
(NOLA.com | The Times-Picayune)

Major voluntary strategies used on Midwest farmland to curb fertilizers that feed the annual low oxygen "Dead Zone" in the Gulf of Mexico don't remove enough nutrients to succeed, according to a new, peer reviewed scientific study.

But combining those strategies with new techniques, including strategically restoring wetlands in some Midwest locations, could reduce nitrogen runoff from farmlands by 45 percent, said the study published in the Journal of the American Water Resources Association.

The Environmental Protection Agency has set a 45 percent reduction goal in nitrogen pollution as necessary to reduce the size of the Gulf dead zone to less than 1,930 square miles. Last August the dead zone was 5,052 square miles, according to measurements by a team of scientists with the Louisiana Universities Marine Consortium.

Federal and state officials along the Upper Mississippi and Ohio river valleys have focused on voluntary measures to carefully control how much fertilizer is used on crops and to plant "cover" crops during offseasons to suck up fertilizer.

The study fund that's not enough, but scientist concluded that the 45 percent is reachable with additional strategies. Those include restoring wetlands in marginal corn and soybean fields and within river and stream paths, as well as reconnecting rivers with their floodplains.

The study, conducted by a team of scientists with the U.S. Geological Survey, U.S. Department of Agriculture and the Environmental Defense Fund, used computer modeling to measure the effects of several nitrogen reduction methods on land segments in the watersheds of the Upper Mississippi and Ohio rivers.

Those watersheds contain much of the nation's intense "farm belt" and soybean agricultural land that have been shown to be the source of the nutrients causing the dead zone.

When spring floods resulting from rainfall and snowmelt wash fertilizer nutrients off the land into the Mississippi River system, the nutrients travel to the Gulf through the mouths of the Mississippi and Atchafalaya rivers.

The freshwater tends to create a layer above salty Gulf water, and the nutrients spawn blooms of algae that eventually die and sink to the Gulf floor. The decomposition of the algae uses up oxygen in the lower, saltier water layer, creating a no-oxygen condition called hypoxia.

The lack of oxygen kills bottom-dwelling organisms that form the base of the fisheries food chain in the Gulf, and cause shrimp and fish to move out of the low-oxygen areas, disrupting commercial fishing.

The consortium of states and federal agencies that make up the Mississippi River Gulf of Mexico Watershed Nutrient Task Force approved a 2008 action plan that called for reducing the size of the dead zone to 1,930 square miles -- or 5,000 spare kilometers -- by 2015.

The plan relied on the voluntary management practices by farmers. But the study's model results indicated that optimizing fertilizer use would only reduce nitrogen pollution by 12 percent, or less than a third of the goal. Adding cover crops increased the reduction level to 30 percent, or two-thirds of the goal.

Adding the other methods the study recommended, however, resulted in significant improvements in the nitrogen reduction numbers. Focusing those methods at areas where they would provide the greatest effects, resulted in both the best reduction results and the least amount of existing farmland taken out of production for wetlands and other reduction strategies, the study found.

"The good news is that adopting soil health and fertilizer efficiency measures across the Corn Belt can get us two-thirds of the way to the tipping point," said the study's lead author and EDF senior scientist, Eileen McLellan, in a news release.

"But by strategically placing wetlands on less than 1 percent of the region's croplands, we'll be able to reverse the trend of significant losses in aquatic life, and improve flood resiliency for downstream communities with minimal impact to crop production," she added.

The additional nitrogen removal practices the scientists recommended also include enhancing drainage ditches to help them hold and remove nutrients. Restoring stream channels and reconnecting floodplains would turn those areas into wetland filters that use marginal land with low crop yields, thus minimizing impacts to crop production, the study said.

The current voluntary strategies of using fertilizer more efficiently and planting cover crops are win-win strategies for farmers, as they reduce fertilizer purchases, the study noted.

Reducing nitrogen levels in some areas can also have the added effect of removing a potential pollution source from public water supplies, scientists wrote.

"The results show that we need to start thinking about conservation not just at the scale of an individual farm but also at the watershed scale," said Dale Robertson, research hydrologist at the Geological Survey's Wisconsin Water Science Center and co-author of the paper. "Improving water quality is a community-wide effort that will save money, clean up local streams, and benefit the Gulf."

The authors of the study point out that there are more than 400 similar low-oxygen water areas around the world that would benefit from following similar nutrient reduction strategies.